In a cellular communication system, such as GSM, each mobile station is identified by an International Mobile Subscriber Identity (IMSI). There is also a public number associated with the IMSI called the Mobile Station International ISDN number or MSISDN. When the MSISDN number is dialled, the home location register (HLR) at which the mobile station is registered translates this number to the IMSI and provides further information about the current location of the mobile station enabling the call to be rerouted to the correct area of the network or even to another network.
It is possible that several different MSISDN numbers can be associated with the same IMSI in a home location register. However, once the IMSI has been identified, the handling of the call is identical in all cases. The unique entry to the IMSI is updated when a mobile station changes location.
Conventional cellular networks can be extended by adding access networks that utilise a low power unlicensed-radio interface to communicate with mobile stations. These access networks are designed to be used together with the core elements of a standard public mobile network and consist essentially of plug-in low-power unlicensed radio transceivers, or access points, each designed to establish an unlicensed radio link with a mobile station MS and a controller or interface node connecting the unlicensed radio transceivers with the mobile core network. Suitable unlicensed-radio formats include digital enhanced cordless telecommunications (DECT), wireless LAN and Bluetooth. An adapted mobile handset capable of operating over both the standard air interface (e.g. the Um interface) and the unlicensed-radio interface means that the subscriber requires only one phone for all environments. The access network is constructed so that the core elements, such as the mobile switching centers MSC, of the public mobile network views the interface node as a conventional base station controller BSC. Such an access network and the mobile station for use with this access network are described in European patent application No. EP-A-1 207 708.
When an unlicensed radio access network is used to provide access to conventional cellular communication networks, the same unique IMSI is used to identify the mobile station to provide seamless mobile services regardless of how the mobile station connects to the network. However, the normal MSISDN number is associated with higher tariffs for the calling party and the called subscriber also has to pay additional costs if the call needs to be routed to another network, as would be the case if the mobile station is used in another country. This is costly for the subscriber and reduces the attraction of an unlicensed radio access network. In many cases there is a need to prevent calls to a mobile via an unlicensed radio access network being redirected to other networks in order to avoid this extra cost.